The present invention relates to a novel method for suspension polymerization which is useful in providing a narrow, small particle size distribution. The method is useful in providing finely divided color developers for use in carbonless paper and cylithographic imaging systems of the type described in U.S. Pat. Nos. 4,399,209 and 4,772,541 in providing toners for electrophotography, and in providing moulding powders.
Methods for the suspension polymerization of acrylates and other unsaturated monomers are well known in the art. The process normally consists in dispensing a liquid monomer in an aqueous phase with stirring to form a dispersion of monomer droplets in the aqueous phase. The suspension is normally stablized by the presence of a dispersing agent, examples of which include gelatin, starch, methyl cellulose, polyvinyl alcohol, salts of polyacrylic and polymethacrylic acids and certain inorganic colloidal materials; e.g., hydrated magnesium silicates and tricalcium phosphate; nonionic surfactants, anionic surfactants and the like.
It is generally considered difficult and expensive to prepare particles in the 1 to 20 micron range. The material being polymerized passes through a sticky, viscous phase during which the particles tend to coalesce causing the formation of aggregates and a lack of uniformity as to particle size and molecular weight. Small amounts of suspension stabilizers or dispersing agents are used to prevent the particles from sticking together and agglomerating during the polymerization process. The lack of suitable stabilizers and dispersing agents is considered a major reason that small particles and a narrow particle size distribution have not been obtained.